Planet Carrier And A Process And Apparatus To Manufacture It

ABSTRACT

The present invention is directed to an integral planet carrier with no joints, a method of manufacturing it and an apparatus for doing so. Such integrally manufactured components have better strength than a conventionally produced multi-piece jointed planet carriers or integral planet carriers made from casting process. The present invention provides a hot forging process which can be used for the manufacturing of the planet carrier. The manufacturing process comprises of forward extrusion of a billet followed by backward extrusion. This is followed by bending operation, which is in turn followed by a flattening operation. Post-forging heat treatment and other treatments such as shot blasting follow. Finally machining is carried out to arrive at the final integrally formed planet carrier. The forward extrusion, backward extrusion, bending and flattening operations are done on a press or hammer having sufficient energy and load capacity. Preferably these operations are performed on a hydraulic press in order to achieve the required accuracy and precision.

FIELD OF INVENTION

The present invention relates to a planet carrier. Particularly, the present invention relates to the planet carrier used in the power transmission system of a vehicle. More particularly the present invention relates to an integral planet carrier and a hot forging process to manufacture the same.

Introduction

A planet carrier is used in epicyclic gear systems. Epicyclic or planetary gear systems consist of three types of gears as given below:

-   -   1. Sun gear (normally only one)     -   2. Planet gears (more than one)     -   3. Ring gear.

This system of gear works in a fashion similar to the planetary system of sun and various planets in the solar system. The sun gear is at the center of the system (similar to sun in the solar system). The planet gears (generally two or more) are in mesh with the sun gear and revolve around it. The ring gear meshes with the planet gears. The rotary motion is transferred from the sun gear to the planet gear to the ring gear or the other way around.

Planet gears are normally mounted on a movable part which is called a carrier. The rotary axes of sun gear and the planet carrier are same but they can rotate independent of each other. The rotational axes of all the gears in the epicyclic gear system are generally parallel to each other (they can be at an angle for some specific cases).

The planetary or epicyclic gear system is used when very high torque transmission or very high transmission ratio is required. During the motion transmission, multiple planet gears are transferring the rotary motion from sun gear to ring gear (or vice versa). The torque or load of transmission is equally distributed among the planet gears. The planetary or epicyclic gear systems are designed to provide high power density in comparison with the standard parallel axis gear trains and are suitable for use in transmission systems of very large dump trucks, tippers etc.

A planet carrier, as explained previously, carries all planet gears. It has two cylindrical parts/sections. As shown in FIG. 1, the larger cylindrical section (1A) is hollow with both its ends closed or covered. One of the closed ends has a central hole through which a shaft can pass. On the other end, a shaft type extension (2) is provided. The hollow cylindrical part (1A) has windows (3) on its side surface. Planet gears are mounted in the hollow cylindrical part (1A) through the windows (3). The sun gear is mounted on a shaft passing through the central hole of the planet carrier. The axes of planet carrier and sun gear are the same while the axes of individual planet gears and the sun gear are parallel to each other.

The planet carrier is used in the transmission system of large tippers or dumpers etc. Due to the complex geometry of the planet carrier, it is normally manufactured in two parts (denoted as part 1 and part 2 in FIG. 1A and FIG. 1B), typically by forging, and then joined together to achieve the final shape. Individual parts that are manufactured separately (Part 1 and Part 2) are then joined together either by welding or bolting. In some cases sheet metal forging process is also used for the manufacturing of individual parts (Part 1 and Part 2).

A number of patents or patent applications disclose planet carriers. However, none discloses an integral planet carrier made using a hot forging method and for use in automotive industry.

Patents CN103615525A and CN104148797B and all disclose methods of manufacturing the planet carrier in two parts and then joining them by EBW or welding or bolting.

Patents U.S. Pat. Nos. 3,667,324, 3,842,481, 4,043,021, 5,558,593, 7,214,160, and 4,721,014 all disclose planet carriers made using sheet metal forming and typically the two plate members that form the planetary carrier are joined together using a method of joining (welding, bolting etc.). None of these patents disclose an integrally formed planetary carrier.

U.S. Pat. No. 9,702,451 discloses a planet carrier that is open in structure and will need closing. Moreover the patent does not disclose a method of manufacture of the planet carrier.

Chinese patent CN103769825 uses a cold-forging method of manufacture of planet carriers which do not have closed structure—it will require a separate closing cover.

Chinese patents CN102829171, CN203809666 and CN204327937 all disclose integrally formed planet carriers that are made using casting. The forged parts have more desirable properties that are not obtained using casting technology.

The Chinese patent 103963233 discloses an integral planet carrier made out of plastic using injection moulding technology. It cannot be applied to metal planet carriers.

Finally, the German patent application DE102011011438 discloses a method of manufacture using Roll Forming Operation that uses incremental forging technique. The planet carrier is not made integrally and involves joining of different parts.

The traditional method as disclosed in prior art has the following drawbacks associated with it:

-   -   1. Since the carrier is made in two parts and then joined, its         strength is inherently less when compared to an integral part as         the strength of the weld depends upon the quality of welding and         its location.     -   2. The manufacturing process becomes lengthy as the two parts         have to be manufactured separately and then a joining process         has to be carried out.     -   3. When Casting method is used for the manufacture of an         integral planet carrier then it will have “as cast”         microstructure which has many casting defects and lower         mechanical properties. The microstructure of this part will         consist of dendrites and casting defects like voids, cracks,         micro porosities etc.

Thus, there exists a room for advancement over the existing technology in that an integral planet carrier and a method of manufacturing the same through hot forging process would not only increase the strength of the part but also reduce manufacturing cycle time.

OBJECTS OF INVENTION

Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:

It is an object of the present invention to provide an integral (single piece) planet carrier.

It is another object of the present invention to provide manufacturing method for the integral planet carrier through hot forging process.

It is still another object of the present invention to provide an integral planet carrier which has better strength to weight ratio.

Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

FIGS. 1A and 1B shows a typical planet carrier of prior art

FIGS. 1C and 1D shows an integral planet carrier of the invention

FIG. 2 illustrates the flow diagram for forging process of the invention

FIG. 3A shows the schematic diagram of set-up used for the forward extrusion process

FIGS. 3B and 3C illustrate the first preform obtained after the forward extrusion process

FIG. 4A shows the schematic diagram of set-up used for the backward extrusion process

FIGS. 4B and 4C illustrate the second preform obtained after the backward extrusion process.

FIG. 5 illustrates the forged integral planet carrier obtained after the flattening operation—FIG. 5A showing the front view while FIG. 5B showing the section view of the forged integral planet carrier.

List of parts 1 - Planet Carrier 1A - Cylindrical part of the planet carrier 2 - Shaft type extension of the planet carrier 3 - Window on the side surface of the cylindrical part (1A) 4 - First preform 4A - Cylindrical part or head of the first preform 4B - Small diameter cylindrical part or a shaft of first preform 5 - Second preform 5A - Walled hollow part or blind hole of the second preform 5B - Small diameter cylindrical part or a shaft of second preform 6 - Third preform 7 - As forged integral planet carrier 7A - Flattened part of as forged integral planet carrier 7B - head portion of the as forged integral planet carrier 7C - Treated integral planet carrier 7D - final integral planet carrier FT - Forward extrusion top die FB - Forward extrusion bottom die BT - Backward extrusion top die BB - Backward extrusion bottom die DT - Bending/deformation top die DB - Bending/deformation bottom die FLT - Flattening top die FU1 - upper cavity in bottom die for forward extrusion (first upper cavity) FL1 - lower cavity in bottom die for forward extrusion (first lower cavity) S1 - upper cavity in bottom die for backward extrusion (first subcavity) S2 - lower cavity in bottom die for backward extrusion (second subcavity) C1 - upper cavity in bending bottom die (second upper cavity) C2 - lower cavity in bending bottom die (second lower cavity) R - Ram R1 - Recess

SUMMARY OF INVENTION

The present invention is directed to an integral planet carrier with no joints and a method of manufacturing the same. Such integrally manufactured components have better strength than a conventionally produced multi-piece jointed planet carrier or an integral planet carrier made from casting process.

The present invention provides a hot forging process which can be used for the manufacturing of the planet carrier. The manufacturing process comprises of forward extrusion of a billet (not shown) followed by backward extrusion. This is followed by bending operation, which is in turn followed by a flattening operation.

The forward extrusion, backward extrusion, bending and flattening operations are done on a press or hammer having sufficient energy and load capacity. Preferably these operations are performed on a hydraulic press in order to achieve the required accuracy and precision.

DESCRIPTION OF THE INVENTION

The present invention relates to the manufacturing of a planet carrier (1) for planetary gear system using the hot forging process. The key inventive feature of this invention is the design and development of the integral planet carrier and the manufacturing process to make the same.

According to the present invention, the manufacturing process starts with a forged billet as a raw material. The raw material is heated to the required temperature in a furnace and then put in forward extrusion dies in a press. As can be seen from FIG. 1, the planet carrier has a large cylindrical section (1A) and a smaller cylindrical section (2). This difference in the diameter between the two sections is produced in the billet during the forward extrusion process (denoted by 4A & 4B in FIG. 3). Thus, the forward extrusion operation gives the required material distribution to the raw material. The forward extruded billet is then transferred to the backward extrusion dies where backward extrusion operation is completed.

The backward extruded preform is then cooled down to room temperature. This preform is heated and then transferred to a forging press where bending operation followed by flattening operation is performed. This operation brings the part to its final form. In another embodiment, the backward extruded preform is transferred directly to bending and flattening stations and the complete process is performed without any intermediate reheating.

The hot forging is followed by a heat treatment process which is followed by the machining process.

Planet Carrier Manufacturing Process

As shown in FIG. 2, the invented manufacturing process typically involves the following steps:

1. Billet or Raw Material Heating

-   -   A forged billet of the required material chemistry is used for         this process. The section and length of the billet taken for         this operation is predefined based on the material requirement         for the part to be produced. The cross section of the billet may         be either round, or rounded cross section (RCS). Preferably a         round cross section billet is used. The billet is heated in an         oil/gas fired or electrical furnace in the temperature range of         1150-1280° C. for sufficient soaking time to achieve uniform         temperature in the heated billet. The output of this process is         a heated billet.

2. Forward Extrusion

-   -   Forward extrusion is a process where the billet is forced to         flow in the same direction as the ram being used to apply         pressure. The forward extrusion process is done using a         combination of a forward-extrusion-top-die (FT) and bottom die         (FB) (see FIG. 3A). The external surface of the         forward-extrusion-top-die connected to a ram is close fitting to         the bottom die cavity consequently preventing material extruding         past it and controlling the flow of the material in the same         direction as the ram.     -   This operation is carried out on the heated billet at a forward         extrusion station using forging equipment having required energy         and load capacity. The heated billet is placed on a bottom die         (FB) aligned concentrically along the central longitudinal axis         of said forward-extrusion-top-die (FT) and subjected to forward         extrusion using the top die-bottom die combination. The forward         extrusion top die (FT) is moved axially and concentrically         towards the said heated billet till said first preform is         produced. This operation imparts the required material         distribution for the backward extrusion step. The output of         forward extrusion operation is a first preform (4), which has a         solid upper cylindrical section or a head (4A), and a solid         lower cylindrical section or a shaft (4B), the diameter of the         solid head (4A) portion being larger than that of the shaft         (4B).     -   The forward-extrusion-top-die (FT) used in the forward extrusion         step is cylindrical in section and has a diameter equal to or         slightly less than the diameter of the heated billet.     -   The bottom die (FB) used in this operation has a cavity with two         diameters. The upper part (or first upper cavity FU1) of the         cavity has a diameter equal to the required diameter of the         solid cylindrical head section 4A and its length/depth is more         than the length of the heated billet by at least 1.5 times.         Further it should be noted that the diameter of the heated         billet used as input to this operation is also equal or slightly         less than the diameter of the upper cavity, so that the heated         billet is easily able to be inserted into the first upper cavity         (FU1). The lower part (or first lower cavity FL1) of the cavity         has a diameter which conforms with the required diameter of the         shaft 4B and its length/depth is 1 mm to 50 mm more than the         required length of the shaft 4B.

3. Backward Extrusion

-   -   The forward extruded first preform (4) is next subjected to the         backward extrusion operation. It is important to minimize the         time of transfer of the first preform (4) to a backward         extrusion station so that the temperature of the first preform         does not reduce any more than 5-10% of its temperature at the         end of the forward extrusion process. Backward extrusion, as the         description suggests is the opposite of forward extrusion and is         where metal is forced to flow in the direction opposite to that         of the ram. The side surface of the backward extrusion top die         (BT) (attached to ram) is recessed circumferentially from its         bottom face up to a length L1 to create a mirror image of the         recess (or a hollow—R1) in the first preform as required. The         material of the head (4A) of first preform flows into this gap         or recess (R1) under the force applied by the movement of         backward-extrusion-top-die (BT) to produce a walled hollow part         (5A) having an open end (second preform—5).     -   In the preferred embodiment, the bottom dies used for the         forward and the backward extrusion steps are the same physical         dies.     -   However, it is possible to use a physically different bottom         die. In this scenario, the bottom die has a cavity which is made         of two subcavities—each subcavity having a different diameter         than the other. The diameter of the first subcavity (S1) is         equal to the required external diameter of the walled hollow         part (5A). The length of the first subcavity (S1) is greater         than the length of the walled hollow part (5A) i.e. L. The         diameter of the second subcavity (S2) is so as to accommodate         the solid cylindrical shaft (5B). The length of the second         subcavity is more than the length of the solid cylindrical shaft         (5B) by 1 mm to 50 mm. In another embodiment, the length of the         second subcavity is equal to the length of the solid cylindrical         shaft (5B).     -   In the case where a physically different bottom die is used for         the backward extrusion operation, the first preform is placed         centrally on the second subcavity such that the central         longitudinal axis of second subcavity and that of the first         preform are aligned.     -   As the ram pushes the backward-extrusion-top-die (BT), the         material of the solid cylindrical head (4A) flows into the         recess (R1)—i.e. gap between the internal wall of the bottom die         (BB) and the backward-extrusion-top-die (BT). The backward         extrusion operation produces a second preform (5) having a         walled hollow part (5A) having an external length L and an         internal length L1, and a solid cylindrical shaft (5B) (see         FIGS. 4B and 4C).     -   The length of the circumferential recess (R1) provided in the         backward-extrusion-top-die (BT) is substantially the same as the         internal length (L1) of the walled hollow part (5A).     -   During the backward extrusion the smaller diameter of the         preform does not change, i.e. diameter of shaft 4B is equal to         diameter of the shaft 5B. This operation can be done on any         forging equipment having sufficient load and energy capacity.         Preferably this operation is done on the hydraulic press. The         output of this process is second preform (5).

4. Heating of the Second Preform

-   -   It is important to maintain the temperature of the second         preform to above a minimum forging temperature at any stage.         Preferably the minimum forging temperature is 900° C. If the         temperature of the second preform falls below this value, the         second preform is heated to the temperature range of         1150-1280° C. It is also possible to simply heat the walled         hollow part (5A) to the temperature range of 1150-1280° C. while         not heating the shaft 5B. The heating of the second preform is         done using induction heater or oil or gas fired furnace. In the         case the second preform is heated, the output of this operation         is heated second preform.

5. Bending

-   -   The bending operation can be done in any forging equipment         having sufficient energy and load capacity. Preferably it is         done on a hydraulic press. The output of this operation is a         third preform (6). In this operation, some portion of the hollow         part or blind hole (5A) of second preform or heated second         preform, near its open end, is deformed (or bent or tilted)         using a bending-top-die towards the central longitudinal axis of         the preform to form an angle with respect to the central         longitudinal axis of the second preform resulting in a tilted         wall.     -   The second preform is placed in a bending bottom die. The         bending bottom die has a cavity (C) with two different diameters         so as to fully accommodate the shaft portion (5B) of the second         preform and partly accommodate the walled hollow part (5A).     -   The lower section (C1)—second lower cavity—of this cavity (C)         conforms to the diameter of cylindrical region 5B of heated         second preform. Further the length of this cavity is equal to or         greater than the length of 5B.     -   The upper section (C2)—second upper cavity—of the bending bottom         die cavity has a diameter conforming to the outer diameter of         the walled hollow part (5A). The length of the upper cavity (C2)         is about 30 to 80% the length (L) of the walled hollow part         (5A). Hence, the hollow part (5A) of second preform projects         above the top surface of the bending bottom die by a length of         projection which is in an amount of 20 to 70% of the external         length L (0.2L to 0.7L—refer to FIG. 4A) of the walled hollow         part (5A). It is the part of the hollow part (5A) which projects         out of the bending bottom die which is bent/tilted inwards (that         is towards the central longitudinal axis) as explained above.     -   Deformation is carried out in the bending operation by moving         the bending-top-die towards the bottom die. The bending-top-die         which has an internal cavity of conical shape with a surface         that is inclined at an angle with respect to the central         longitudinal axis of the bending top die—is pushed towards the         second preform. The angle is less than 90°, preferably between         15° to 55° when measured from the central longitudinal axis. As         it comes in contact with the second preform, the part of the         second preform that sticks out above the bending bottom die         deforms under the force applied due to the movement of the top         die. The output of this operation is third preform (6). The         remaining non-deformed part of the hollow part (5A) remains         substantially parallel to the longitudinal axis of the third         preform (6).     -   The smaller diameter part of the third preform (6) remains         undeformed in this operation also.

6. Flattening

-   -   The bottom die for the flattening operation is same as that used         for the bending operation. Only the top die is replaced for the         flattening operation by a flattening-top-die which has an         annular second recess or ring shaped cavity. The outer diameter         of this ring shaped cavity is equal to the outer diameter of the         head portion (7B) of as forged integral planet carrier (7). The         inner diameter of this ring shaped cavity is equal to the inside         diameter D (as shown in FIG. 5B) of 7A. In the flattening         operation the deformed/bent/tilted wall portion of third preform         (6) is further deformed to flatten it to form a flattened part         (7A). The flattened surface is substantially perpendicular to         the longitudinal axis. The smaller diameter part of the preform         remains undeformed in this operation also. The output of this         operation is the hot-forged integral planet carrier (7) as shown         in FIG. 5.

7. Heat Treatment and Post Forging Operation

-   -   The hot-forged integral planet carrier (7) thus produced is then         heat treated to achieve the required mechanical properties.         Post-forging operations like shot blasting etc. are also carried         out as appropriate on the part. The output of this operation is         treated integral planet carrier.

8. Machining

-   -   Sections of the hot forged integral planet carrier are carved         out and machined so that a final integral planet carrier as         shown in FIGS. 1(C and D) is produced.

The process disclosed herein thus produces a planet carrier (1) which is an integral or integrally formed object, devoid of joints and is made using hot forging technique.

As another aspect of the invention, the apparatus to produce an integrally formed planet carrier is disclosed. It comprises the following tools placed sequentially:

-   -   a billet heating station for heating the billet;     -   a forward extrusion station to forward extrude said billet into         the first preform (4);     -   a backward extrusion station to backward extrude said first         preform (4) into the second preform (5);     -   a bending station to deform the walled hollow part (5A) of said         second preform (5) to produce a third preform (6) having a         deformed or bent or tilted walled hollow part;     -   a flattening station for flattening said deformed or bent or         tilted walled hollow part to produce the as-forged integral         planet carrier (7);     -   a heat treatment and post-forging treatment station for treating         said as-forged integral planet carrier into the treated integral         planet carrier (7C);     -   a machining station to machine said treated integral planet         carrier (7C) into said final integral planet carrier (7D).

It is evident from the foregoing discussion that bending station disclosed here requires a forging equipment, preferably a hydraulic press, in which are placed a bending top die (DT) and a bending bottom die (DB), both dies placed co-axially, and wherein said bending top die (DT) has a conical cavity that faces said bending bottom die (DB), wherein when said second preform (5) is placed on said bending bottom die (DB), said bending top die (DT) is capable of moving towards said bending bottom die (DB) so as to accommodate the projection of said second preform (5). Further, the surface of said conical cavity has an angle less than 90 degrees when measured from its central longitudinal axis, and preferably between 15 and 55 degrees. In another aspect of the apparatus disclosed here, the flattening station has a flattening top die (FLT) having an annular second recess or a ring shaped cavity. Finally, a heating station is provided between the backward extrusion stations and the bending station for heating said second preform (5), if necessary, to ensure that the temperature of the second preform doesn't fall below the minimum required forging temperature.

The benefits of this invention are as follows:

-   -   1. In this invention a manufacturing process has been proposed         which allows the manufacture of the planet carrier using         bulk/hot forging method without any joints. Thus, the output of         the process is an integral planet carrier with excellent product         properties achieved due to hot forging process (shown in FIGS.         1C & D).     -   2. The integral nature of the part improves the strength of the         part which increases its life.     -   3. The forged structure of the part (i.e. continuous grain flow         lines, equiaxed grains/microstructure, directional properties         and absence of any voids, micro porosities or cracks) also         improves the mechanical and fatigue properties of the part and         hence, increases its life.     -   4. The inventive feature of the forging process flow is the         sequential forging steps and the forging dies which lead to         formation of the enclosed or covered structure which is formed         in as forged condition (shown in FIG. 5B) eliminating the need         for use of any joining method like welding, bolting etc.     -   5. The apparatus disclosed here allows production of a planet         carrier which is an integral or integrally formed object, devoid         of joints and is made using hot forging technique

While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents. 

1. A planet carrier, characterised in that said planet carrier is an integral planet carrier (7D), devoid of joints and is made using hot forging, said planet carrier (1) comprising a cylindrical part or a head (7B) which has a flattened part (7A), and a shaft (5B), and wherein sections of said planet carrier (1) are carved out and machined to produce a final integral planet carrier (7D).
 2. A planet carrier as claimed in claim 1 characterised in that it has continuous grain flow lines.
 3. A process of making an integral planet carrier (7D), characterised in that said process comprises the steps of: heating of a billet to produce a heated billet; forward extruding said heated billet to produce a first preform (4) having a head (4A) and a shaft (4B); backward extruding said first preform (4) to produce a second preform (5) having a walled hollow part (5A) and a shaft (5B); ensuring that the temperature of the second preform (5) is maintained at or above a minimum forging temperature; bending or deforming walled hollow part (5A) of said second preform (5) to produce a third preform (6) having a head with a bent or deformed or a tilted wall portion; flattening said bent or deformed or tilted wall portion to produce a hot forged integral planet carrier having a head (7B) with a flattened part (7A); heat treating said hot forged integral planet carrier (7) having a flattened head (7A) followed by shot blasting it to produce treated integral planet carrier (7C); machining of said treated integral planet carrier (7C) to produce final integral planet carrier (7D); said process thus leading to an integrally formed planet carrier (7D).
 4. A process as claimed in claim 3, characterised in that in the step of heating, said billet is heated in a furnace to a temperature of 1150 to 1280° C.
 5. A process as claimed in claim 3, characterised in that said forward extruding step is carried out at a forward extrusion station using a combination of a forward-extrusion-top-die (FT) and a bottom die (FB), wherein said heated billet is placed on said bottom die (FB) aligned concentrically along the central longitudinal axis of said forward-extrusion-top-die (FT) and said forward extrusion top die (FT) is moved axially and concentrically towards the said heated billet till said first preform (4) is produced which has a solid upper cylindrical section or a head (4A), and a lower cylindrical section or a shaft (4B), wherein the diameter of the head (4 A) portion is larger than that of the shaft (4B).
 6. A process as claimed in claim 3, characterised in that said backward extruding step is carried out in an hydraulic press using a combination of a backward-extrusion-top-die (BT) and a backward extrusion bottom die (BB), wherein the side surface of the backward extrusion top die (BT) is recessed circumferentially from its bottom face up to a length (L1) thereby creating a recess or gap (R1) between said side surface and the internal face of the upper cavity of said backward extrusion bottom die (BB), and wherein material of said head (4A) flows into said gap (R1) under the force applied by the movement of said backward-extrusion-top-die (BT) to produce a walled hollow part (5A) having an open end as a part of said second preform (5).
 7. A process as claimed in claim 3, characterised in that in the step of ensuring that said second preform (5) remains over a minimum forging temperature, second preform (5) is heated if its temperature falls below said minimum forging temperature, heating being carried out preferably to a temperature between 1150 and 1280° C.
 8. A process as claimed in claim 3, characterised in that said bending step to produce said third preform (6) is carried out on a forging equipment, preferably a hydraulic press using a bending top die (DT) and a bending bottom die (DB), wherein when the second preform (5) or heated second preform (5), as the case may be, is placed on said bending bottom die (DB), such that there is a projection of said walled hollow part (5 A) that projects above the top surface of said bending bottom die (DB).
 9. A process as claimed in claim 8, characterised in that said bending top die (DT) has an internal cavity of conical shape with a surface that is inclined at an angle with respect to the central longitudinal axis of said second preform (5).
 10. A process as claimed in claim 8, characterised in that said angle is less than 90 degrees.
 11. A process as claimed in claim 8, characterised in that said angle is between 15 and 55 degrees.
 12. A process as claimed in claim 8, characterised in that the length of projection is 20% to 70% of the external length of said walled hollow part (5A).
 13. A process as claimed in claim 8, characterised in that said bending-top-die (DT) is pushed towards the second preform (5) until said projection is substantially within said conical cavity, thereby forming a third preform (6) that has said bent or tilted wall portion.
 14. A process as claimed in claim 3, characterised in that said flattening operation performed on said bent or tilted wall portion of said third preform (6) is carried out using the same bottom die as used in said bending step, and wherein a flattening top die (FLT) having an annular or ring shaped cavity is used for carrying out the flattening of said tilted wall portion to produce a hot-forged integral planet carrier (7).
 15. A process as claimed in claim 3, characterised in that said hot-forged integral planet carrier (7) is heat treated and subjected to post-forging treatments such as shot-blasting to produce a treated integral planet carrier (7C).
 16. A process as claimed in claim 3, characterised in that said treated integral planet carrier (7C) is machined to produce a final integral planet carrier (7D).
 17. A process as claimed in claim 3, characterised in that said minimum forging temperature is 900° C.
 18. A process as claimed in claim 3, characterised in that in the step of ensuring that said second preform remains over a minimum forging temperature, the walled hollow part (5A) of said second preform (5) is heated if the temperature of said second preform (5) falls below said minimum forging temperature, heating being carried out preferably to a temperature between 1150 and 1280° C.
 19. A hot forging apparatus to make a planet carrier (7D), characterised in that said apparatus comprises the following tools placed sequentially: a billet heating station for heating said billet; a forward extrusion station to forward extrude said billet into said first preform (4); a backward extrusion station to backward extrude said first preform (4) into said second preform (5); a bending station to deform said walled hollow part (5 A) of said second preform (5) to produce a third preform (6) having a deformed or bent or tilted walled hollow part; a flattening station for flattening said deformed or bent or tilted walled hollow part to produce said as-forged integral planet carrier (7); a heat treatment and post-forging treatment station for treating said as-forged integral planet carrier into a treated integral planet carrier (7C); a machining station to machine said treated integral planet carrier (7C) into said final integral planet carrier (7D).
 20. A hot forging apparatus as claimed in claim 19, characterised in that said bending station comprises a forging equipment, preferably a hydraulic press, in which are placed a bending top die (DT) and a bending bottom die (DB), both dies placed co-axially, and wherein said bending top die (DT) has a conical cavity that faces said bending bottom die (DB), wherein when said second preform (5) is placed on said bending bottom die (DB), said bending top die (DT) is capable of moving towards said bending bottom die (DB) so as to accommodate the projection of said second preform (5).
 21. A hot forging apparatus as claimed in claim 19, characterised in that surface of said conical cavity has an angle less than 90 degrees when measured from its central longitudinal axis.
 22. A hot forging apparatus as claimed in claim 19, characterised in that said angle is between 15 and 55 degrees.
 23. A hot forging apparatus as claimed in claim 19, characterised in that said flattening station has a flattening top die (FLT) having an annular second recess or a ring shaped cavity.
 24. A hot forging apparatus as claimed in claim 19, wherein a heating station is provided between said backward extrusion station and said bending station for heating said second preform (5). 